Revise initShader()

main.cpp

Because we are going to use multiple shaders, we like initShader() to return the shader id. Accordingly, we are changing the return value of initShader() from void to GLuint.

GLuint initShader(std::string pathVert, std::string pathFrag) 
{
    shader.read_source( pathVert.c_str(), pathFrag.c_str());

    shader.compile();
    glUseProgram(shader.program);

    return shader.program;
}